The B lymphocyte (B cell) is one of the most important cells of the body. They form a part of the adaptive immune response by producing antibodies and presenting antigens to T cells. Once activated they will either mature into plasma cells or memory B cells.
This article will focus on the development of B cells and the main types found within the body.
B Cell Development
The cells that give rise to B and T lymphocytes are found within the bone marrow. Pre-T cells migrate to the thymus, whereas those developing into B cells remain in the bone marrow. The progenitor cells that are committed to the formation of B cells (as proposed to the other types e.g. T cells and NK cells) are chosen at random.
B cells remain in the bone marrow while maturing and during this process any that respond to self-antigens undergo apoptosis – this prevents autoimmune reactions from occurring once they enter the peripheral circulation.
Once differentiated in the primary lymphoid organ (bone marrow), the B cells migrate and reside in the follicles of the spleen. They also migrate to areas that are rich in lymphoid tissue – areas where lymphoid activation and defense is likely to be triggered, e.g. in the mucosa and linings as associated lymphoid tissue. An example of this is the Peyer’s patch of the colon, which is a mucosally associated Lymphoid tissue (MALT). Other ‘ALTs’ also exist, their names according to their locations e.g. Bronchal (BALT), Nasal (NALT), Organised-mucosa (O-MALT).
Types of B Cell
Once activated B cells may differentiate into plasma cells. Plasma cells are large lymphocytes with large amounts of endoplasmic reticulum, which allows them to produce antibodies to specific antigens.
They respond to signals from T cells during infection and continue to produce antibodies to the required antigen until the infection is controlled. Plasma cells are often found within chronic infection and inflammation.
Memory B Cell
Other B cells will differentiate into memory B cells when activated. These are long lived cells which remain within the body and allow the body to respond much more rapidly in the case of a subsequent infection.
If the host is re-exposed to the same antigen these cells rapidly divide with assistance from T cells and produce more B cells capable of producing specific antibodies to the pathogen. This often means that the pathogen can be dealt with before the infection takes hold.
T-independent B Cells
Most B cells require T cells to be present in order to produce antibodies, however a small number are able to function without this. They are found within specialised sites such as the spleen and peritoneum.
They are particularly important for dealing with encapsulated bacteria, which often have a polysaccharide outer layer as opposed to proteins, which allows them to evade T cells. T-independent B cells are able to recognise these layers and produce antibodies to them without the need for T cell help.
XLA, also known as Bruton’s disease, is a rare genetic disorder that affects the body’s ability to fight infection. Patients are unable to produce mature B cells, as such they tend to have an absence of serum immunoglobulins post 6 months (after maternal IgG have been broken down).
Like many primary immune deficiencies, patients typically present with infections that are severe, persistent, uncommon and recurrent. H. influenzae, Streptococcus pneumoniae, and staphylococci are the most common causes.
It is generally diagnosed by genetic testing.
Treatment is with immunoglobulin replacement therapy and patients may also require prophylactic antibiotics. Patients with XLA should not receive live vaccines.